1. Field of the Invention
The present invention relates to an apparatus and method for fabricating a powdery thermoelectric material in order to fabricate a thermoelectric module that performs the conversion between thermal energy and electric energy.
2. Description of the Related Art
A xe2x80x9cthermoelectric phenomenonxe2x80x9d is the general term of the Seebeck effect, the Peltier effect and the Thomson effect, and elements utilizing the phenomenon are called a xe2x80x9cthermoelectric elementxe2x80x9d, a xe2x80x9cthermocouplexe2x80x9d, an xe2x80x9celectronic cooling elementxe2x80x9d, etc. The thermoelectric phenomenon was originally discovered between different kinds of metals, but in recent years, thermoelectric materials of semiconductors have come to be obtained, and conversion efficiencies not observed with metal materials have come to be attained. Elements employing the thermoelectric semiconductor materials are structurally simple and easy of handling, and can maintain stable characteristics, so that their uses in a wide range attract public attention. In particular, since the elements are capable of precise temperature controls at and near the room temperature, researches and developments have been extensively promoted for temperature regulations in optoelectronics, semiconductor lasers, etc., and for applications to local cooling, small-sized refrigerators, etc.
In the fabrication of the thermoelectric element, a method of weighing capacity of raw materials to a desired composition, preparing a solid solution ingot by heat-melting and solidifying, further powdering the solid solution ingot and then sintering, slicing and dicing the same has been adopted so far. As a method of powdering the thermoelectric material in the process described above, there is a method of pulverizing the solid solution ingot and the resulting powder is classified by sieving. With this method, however, in order to pulverize solidified solid material, the powdery grains are in the shape of flakes. Therefore the loading of a sieve at the process of the classification and decrease of filling rate in the case where a die is filled up with the powder in order to mold it in the process of compressing the powder are occurred. In order to resolve such problems, there is a method of using a globular powdery thermoelectric material to fabricate a thermoelectric element. For example, Japanese Publication of Unexamined Patent Application, No.293276/1992 discloses a method of fabricating a globular powdery thermoelectric material. Heretofore, the globular powdery thermoelectric material has been obtained by a method referred to as a rotating disk method (or a centrifugal atomization method) of mixing and melting predetermined raw materials, and dropping to scatter the obtained molten metal onto a rotating disk manufactured from a metal (material) or a ceramic (material).
By the way, when a thermoelectric module is fabricated by using a globular powdery thermoelectric material, it has been known that the smaller the diameter of the powder is, the better performance of the module can be attained. In view of the above, for fabricating a fine powdery thermoelectric material with a diameter, for example, of 40 xcexcm or less, the disk has to be rotated at a high speed.
In order to obtain a powdery thermoelectric material by a disk rotating at high speed, the disk has to satisfy various conditions. That is, the disk is imposed conditions that (1) it has a reduced weight and a sufficient mechanical strength so as to withstand high speed rotation, (2) it has heat resistance and thermal shock resistance capable of withstanding high temperature of the heat-melted thermoelectric material and has small coefficient of thermal expansion, (3) heat capacity of the entire disk is small in order to prevent the molten metal of the thermoelectric material from solidifying on the disk, and (4) the disk has less reactivity with the molten metal of thermoelectric material so as to avoid contamination of impurities into the thermoelectric material.
However, since conventional disks have large disk diameter and mass, it is difficult to rotate them at high speed. In addition, since metal or ceramic is used for the material of the disk, the heat capacity of the disk is large so that the molten metal of the thermoelectric material is deprived of its heat by the disk and the molten metal is easy to solidify on the disk. As a result, the disk is further increased in the weight and difficult to be rotated at high speed, and it is easy to get off the rotational balance of the disk. Further, this lowers the yield of the powdery thermoelectric material.
In order to improve these points (subjects), for example, when the weight of the disk is decreased for enabling high speed rotation and reducing the heat capacity, mechanical strength is lost since the thickness of the disk is decreased. On the contrary if it intends to maintain the mechanical strength, the inertial mass and the heat capacity of the disk are increased. Further, when a metal (material) is used as the material for the disk, since the coefficient of thermal expansion is large, the material is strained by thermal stress to possibly worsen the durability. Particularly, when iron or titanium is used as the material, since the material is highly reactive with the molten metal of the thermoelectric material, the composition of the thermoelectric material is changed. As described above, materials for a disk capable of satisfying all of said conditions have not yet been found so far.
In view of the above, it has been also practiced to manufacture a disk by a combination of two kinds of materials. For instance, Japanese Publication of Unexamined Patent Application No. 145710/1990 discloses a structure in which a metal disk is covered with a heat insulating material and the circumference thereof is held by a metal holder. Further, Japanese Publication of Unexamined Patent Application No. 34102/1995 discloses a structure in which a ceramic layer is disposed on the surface of a lightweight titanium alloy. However, in the conventional structures described above, since the disk is relatively larger, it can not be rotated at a high speed and the maximum rotational speed is, for example, at about 15,000 rpm, and the minimum grain size particle diameter can be reduced to no more than about 130 xcexcm as well. Further, there is still left a problem that the molten metal is deprived of its heat by the disk and is easy to solidify on the disk to lower the powder yield.
In view of the foregoings, this invention intends to provide a method of fabricating a powdery thermoelectric material and a manufacturing apparatus therefor capable of fabricating a fine powder at a high yield, when fabricating a powdery thermoelectric material by a rotating disk method, by adopting a disk using a material having a reduced weight and a high strength, with lower thermal expansion coefficient and less reactivity with the material, and designed so as to decrease the heat capacity, thereby preventing the molten metal from solidification and capable of fabricating a powder at high speed rotation. In view of the foregoing, it is an object of the present invention to provide a method of fabricating powdery thermoelectric material and an apparatus for fabricating powdery thermoelectric material which can prevent from solidifying the molten metal and fabricate a fine powder at a high yield, because of using the disk manufactured by the material which has lightweight, high mechanical strength, lower coefficient of thermal expansion and less reactivity with the materials and designed to less heat capacity when fabricating a powdery thermoelectric material by a rotating disk method.
The foregoing subjects can be solved in accordance with the present invention by an apparatus for fabricating a powdery thermoelectric material comprising a container for mixing raw material having a predetermined composition and heating and melting the same, a funnel or a pouring port for pouring the molten metal of the heated and melted raw material and a rotating disk made of silicon nitride or a material containing silicon nitride for scattering the poured molten metal. In order to solve the subjects mentioned above, an apparatus for fabricating a powdery thermoelectric material related to the present invention comprises a container for mixing and heat-melting a raw material of predetermined composition, a funnel or a pouring port for pouring the heat-melted raw material, and a rotating disk made of silicon nitride or a material containing silicon nitride for scattering the poured molten metal. The rotating disk may be manufactured from a material containing 90% or more of silicon nitride.
Further, the foregoing subjects can be solved in accordance with this invention by a method of fabricating a powdery thermoelectric material comprising mixing a raw material having a predetermined composition and heating and melting the same, pouring the molten metal of the heated and melted raw material on a rotating disk manufactured from silicon nitride or a material containing silicon nitride and scattering the poured molten metal by a rotating disk into fine globular forms and cooling them. Further a method of fabricating a powdery thermoelectric material which related to the present invention comprises the step of mixing and heat-melting a raw material of predetermined composition; the step of pouring the heat-melted raw material onto a rotating disk made of silicon nitride or a material containing silicon nitride; the step of turning the heat-melted material into microglobules by scattering and then cooling the microglobules, thereby to prepare a globular powdery thermoelectric material. The said rotating disk may be manufactured from a material containing 90% or more of silicon nitride.